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One word to denote a person who derives pleasure from another's misfortune?
One who feels or expresses triumphant and malicious satisfaction at another's misfortune.
Sadist: One who derives pleasure through cruelty or pain to others.
The literal definition has a strong physical connotation to it, but the term does often get used loosely.
The a word schadenfreude borrows from the German Schadenfreude (“joy in the misfortune of others”).
There is also a rare word schadenfreuder, a person experiencing schadenfreude.